Electromagnetic noise can interfere with the normal operation of electric devices or electric machines, which is commonly referred to as electromagnetic interference (EMI). When EMI is present, these electric devices may not operate reliably or efficiently.
In electric motors, a motor drive may be provided that includes a converter that switches power, such as high voltage power, on and off. The switching operation can result in EMI. For example, pulse width modulation (PWM) converters are commonly used in various applications due to the control performance and high efficiency characteristics of the PWM converters. However, the switching pattern of the power semiconductor devices in the converter induces a common-mode (CM) current which is coupled to the ground through the load parasitic capacitor. In some applications, an un-shielded cable also may have to be used to connect the load and the converter. As a result, CM noise can be a significant issue because there is no longer a low impedance path to bypass the ground current.
Some known systems add additional wires and/or CM inductors into the cable bundle to achieve the desired CM noise filtering. However, these known arrangements often have to include large choke devices which increase the size, weight and cost of the system. Other known systems inject CM current onto the DC input side of the device. However, the CM current at the AC output of the device cannot be reduced. Still other known systems provide a controlled voltage source at the AC output to cancel the CM current. However, in these systems, additional active devices and a transformer in series with the main AC cable are required.